Conventional antenna mounts are normally required to mechanically steer high-gain antenna systems in two dimensions. In some mobile applications, such as ship-mounted antennas, the required steering range can be up to full hemispheric; however, in other applications, e.g. forward-looking radar antennas in the nose of aircraft, the steering range is limited to a narrower region. Similarly, multiple shipboard antennas, each with limited steering range, which in combination cover a large steering range, are disclosed in a paper by E. Barry Felstead, entitled “Combining multiple sub-apertures for reduced-profile shipboard satcom-antenna panels,” in Proc. IEEE Milcom 2001, unclassified paper 19.6, Vienna, Va., 28-31 Oct. 2001; and in a paper by E. Barry Felstead, Jafar Shaker, M. Reza Chaharmir and Aldo Petosa, entitled “Enhancing multiple-aperture Ka-band navy satcom antennas with electronic tracking and reflectarrays,” in Proc. IEEE Milcom 2002, paper U105.7, Anaheim, Calif., 8-10 Oct. 2002.
Regardless of the application, the steerable antenna mounts are preferably made as compact as possible by minimizing the size of the motors, and the profile depth, mass, and volume of the combined antenna and mounting structure. Moreover, it is also desirable to make the antenna mounts relatively simple and inexpensive to build.
Steering or pointing of the antenna involves a rotation about a single axis or about a plurality of axes, e.g. a variety of different axes used in various combinations depending upon the application of the antenna. Typically, the basic axes are referred to as azimuth, elevation, cross-elevation, and cross-level, as is well known in the art. Driving motors are usually used for actuating the rotation about the different axes. The different axes can be coupled together in a variety of ways including the use of gimbals.
With reference to FIG. 1, for discussion purposes, the reference coordinates are (xr,yr,zr) with the antenna system located at (0, 0, 0). The zenith is considered to be in the direction of the zr axis, and the xr and yr coordinates lie in the horizontal plane. For mobile applications, the yr axis could be pointed in the direction of forward motion. Spherical coordinates (φ,θ,ρ) are also illustrated in FIG. 1, in which the angle φ corresponds to the azimuth angle, and the angle θ corresponds to the complement of the elevation angle, ε, i.e. ε=90°−θ.
With reference to FIG. 2(a), a common two-axis antenna mount is an elevation-over-azimuth mount 1 for antenna 2, which uses a first motor (not shown) providing up to full azimuth rotation (360°) about a vertical axis V, and a second motor (not shown) providing full elevation rotation (90°) about a horizontal axis H. The center of gravity of the antenna 2 is usually offset from the pivot points, thereby requiring that the first and second motors have increased torque. These disadvantages can be reduced in certain applications in which the elevation range of the antenna is more limited, such as with the KVH series of satellite-dish antennas. Corey Pike and Claude Desormeaux, disclosed the adaptation of a type G3 KVH antenna for a vehicle-mounted application in the reference entitled “Ka-band land-mobile satellite communications using ACTS”, 7th Ka-Band Utilization Conf., September 2001, and Richard S. Wexler, D. Ho, and D. N. Jones, disclosed the adaptation of a type G6 by MITRE in the reference entitled “Medium data rate (MDR) satellite communications on the move (SOTM) prototype terminal for the Army warfighters,” in Proc. IEEE Milcom 2005, Atlantic City, Oct. 17-20, 2005. Unfortunately, the elevation-over-azimuth mount also has problems with cable wrap and with the keyhole effect in the zenith direction, as will be discussed later.
A less-common type of mount is the cross-elevation-over-elevation mount 5, as illustrated in FIG. 2(b), sometimes referred to as an “X-Y” mount. An elevation motor (not shown) is used to rotate an antenna 6 about a first horizontal elevation axis, and a cross-elevation motor (not shown) is used to rotate the antenna 6 about a cross-elevation axis. The mass of both the antenna 6, and the cross-elevation motor must be supported by the elevation motor, thereby adding to the motor-torque requirements; however, the X-Y mount does not have a keyhole problem in the zenith direction and does not have a cable wrap problem. Unfortunately, the X-Y mount tends to have a reduced steering range compared to the elevation-over-azimuth mount.
In certain applications, such as on naval ships, a third axis of steering is sometimes added to the antenna mount to get around the keyhole problem that the standard azimuth-elevation mount exhibits in the zenith direction. Another purpose is to add what is sometimes called a “cross-level” axis to simplify the compensation for ship roll and pitch.
An alternative approach to antenna steering is disclosed in U.S. Pat. No. 6,911,950 issued Jun. 28, 2005 to Harron, referred to as the “universal-joint gimbaled antenna mount” (or the “GiAnt” mount). As illustrated in FIG. 3, the antenna 7 plus the feed system is placed so that the center of mass is at, or near, the center of the universal joint 8, such as a ball joint. A yoke 9 driven by a motor (Motor 2) scans the antenna 7 about the elevation axis EA, and another motor (Motor 1) mounted on the yoke 9 pivots the antenna 7 about the ends of the yoke 9, i.e. scans the antenna 7 about the cross-elevation axis XEA. Since the center of mass of the system rests on the ball joint 8, the motors (Motor 1 and Motor 2) can be very small, i.e. small digitally driven stepper motors with built in shaft encoders can be use. Such a system can scan to over ±50° in both elevation and cross elevation, and with careful mechanical design could be slightly extend. As a result of the “X-Y” form of scanning, there is no problem with cable wrap, and the keyhole has been pushed far from boresight. Moreover, the GiAnt mounting system is relatively inexpensive to manufacture.
Unfortunately, the GiAnt mounting structure exhibits vibration in the form of twisting of the yoke 9 when mounted on a platform undergoing severe movements, e.g. ship mounted. The yoke 9 could be strengthened, but difficulties arise when making it sufficiently rigid for the steering accuracies likely to be encountered.
Rotating-wedges were disclosed by G. Maral and M. Bousquet, in Satellite Communications Systems: systems, Techniques and Technology, Fourth ed., by John Wiley & Sons, Chichester UK, 2002, pages 392 to 394, for supplementing a standard steering system to give a slight offset “bias”, which is used to avoid the keyhole problem, but were not intended to be used as the means of steering in one of the major axes.
An object of the present invention is to overcome the shortcomings of the prior art by providing an antenna steering mount comprised of two counter-rotating wedged bodies.